结构打字需要很少的帮助

Question

I noticed the repeating pattern in my code, and thought this may be a good idea to give structural typing a try. I've read the chapter ;-), but I can't quite get my head around it. Consider the following code:

  def link(user: User, group: Group) = {
    UserGroupLinks.insert((user.id, group.id))
  }

  def link(group: Group, role: Role) = {
    GroupRoleLinks.insert((group.id, role.id))
  }

  def link(user: User, role: Role) = {
    UserRoleLinks.insert((user.id, role.id))
  }

How do I combine it into something like:

def link(A <: ...something with an id, B<:... something with and id) = {
  (A, B) match {
    case (u: User,  g: Group) =>  UserGroupLinks.insert((user.id, group.id))
    case (g: Group, r: Role)  =>  GroupRoleLinks.insert((group.id, role.id))
    case (u: User,  r: Role)  =>  UserRoleLinks.insert((user.id, role.id))
    case _ =>
  }
}

Answer 1

With structural types it would be something like this:

type WithId = {def id:Int}

def link(a:WithId, b:WithId) =
  (a, b) match {
    case (u:User, g:Group) => UserGroupLinks.insert(u.id -> g.id)
    case _ =>
  }

---

Edit

You could go a bit further and let the compiler help you with selecting the correct inserter. For that you would need to introduce a trait on your inserters:

trait WithInsert[A, B] {
  def insert(x: (Int, Int)): Unit
}

And then on your insert objects do this:

object UserGroupLinks extends WithInsert[User, Group]

You can define the default ones on the companion object

object WithInsert {
  implicit val ug = UserGroupLinks
  implicit val gr = GroupRoleLinks
}

We can still use the WithId type although in most cases I would recommend using a trait

type WithId = { def id: Int }

Your link method then would look like this:

def link[A <: WithId, B <: WithId](a: A, b: B)(implicit inserter: WithInsert[A, B]) =
  inserter.insert(a.id -> b.id)

As you can see the link method expects a WithInsert[A, B] to be available. It will find the appropriate one in the companion object of WithInsert .

That means you can now call your method simply like this:

link(user, group)
link(group, role)

Answer 2

This is the simple solution I was looking for:

  def link[A <: { def id: Long }, B <: { def id: Long }](a: A, b: B) = {
    (a, b) match {
      case (u: User, g: Group) => UserGroupLinks.insert((u.id, g.id))
      case (g: Group, r: Role) => GroupRoleLinks.insert((g.id, r.id))
      case (u: User, r: Role)  => UserRoleLinks.insert((u.id, r.id))
      case _ => ???
    }
  }

Where the compiler would allow:

case class XX(id:  Long)
case class YY(id:  Long)
case class ZZ(idz: Long)

link(XX(22), YY(33))

But not:

link(XX(22), ZZ(33))

Answer 3

Try

type T = { val id: String }

def link(obj1 : T, obj2: T) = {
   // You got 
   println(" obj1.id = %s obj2.id = %s".format(obj1.id, obj2.id))  
}